Speaking in 1949, the year in which the first Royal Commission on the Press rejected the notion that newspapers might be improved by state intervention, Sir Winston Churchill said:

"A free Press is the unsleeping guardian of every other right that free men prize; it is the most dangerous foe of tyranny ... Under dictatorship, the Press is bound to languish ... But where free institutions are indigenous to the soil and men have the habit of liberty, the Press will continue to be the Fourth Estate, the vigilant guardian of the rights of the ordinary citizen."

That has been the settled view in Britain throughout the democratic era, but as they anticipate Lord Justice Leveson's recommendations for "a new and more effective way of regulating the Press" this week, many citizens may feel tempted into sympathy for a substantially less liberal attitude towards newspaper journalism.

It would take a hard heart to deny that the murdered schoolgirl Milly Dowler and her father, mother and sister, Bob, Sally and Gemma, were atrociously treated by the News of the World.

Only a fool could defend newspaper treatment of Gerry and Kate McCann, whose daughter Madeleine disappeared in 2007, of Christopher Jefferies, Bristol landlord of the murdered Joanna Yeates, and of the singer Charlotte Church.

Add to these blameless individuals celebrity hacking targets including Elle Macpherson and JK Rowling and the case for what the Hacked Off pressure group wants begins to appear unassailable. At least, it does until you think about it. Scrutiny exposes its case as offensive to the principle of free speech.

Hacked Off asserts that a "free and accountable media" can be achieved by "independent regulation of the Press backed by law". It is an oxymoron: in what sense can newspapers be independent if they are regulated by statute?

Proposals advanced in support of the campaign by the Media Standards Trust confirm that they can't. The trust proposes self-regulation backed by statute and supervised by an independent auditor with statutory authority.

Hacked Off and the Media Standards Trust are determined to blur the distinction between statutory regulation and self-regulation by pretending that they are not really different at all.

They overlook the conclusions of three Royal Commissions on the Press and the policies of every peacetime government.

Whether or not Lord Justice Leveson is persuaded by their arguments when he publishes his long-awaited report on Thursday, the Government should not endorse them. Above all, parliamentarians must remember that Leveson has responsibility to advance proposals, but no power to make them law.

The news industry is often raucous and impertinent, yet it stands in a quasi-constitutional relationship to government. It informs, investigates and analyses on the public's behalf.

Since newspaper journalism holds government to account, government must not regulate newspapers.

The constitutional objection is plain: if politicians regulate newspapers, they will make sure they get the Press they want, not the Press they deserve. This is why that trifling instrument of democracy, the Constitution of the United States, declares: "Government may make no law abridging the freedom of the Press."

Maintaining independence of newspapers from the state is important in Britain because here executive and legislature are not separate. Our ministers sit in the House of Commons and lead a parliamentary majority. This hybrid arrangement gives a British government power to ensure its legislation is passed; a level of executive power absent from other democratic traditions.

To balance it, we have developed a system in which additional checks are exercised in the public interest by the courts and the Press. Statutory regulation of newspapers would create a constitutional absurdity: government authority over a body the electorate depends upon to scrutinise government.

Zealous supporters of statutory underpinning pretend that this danger is non-existent or worth ignoring. Their more sophisticated sympathisers insist that fears about a statutory backstop to Press regulation are exaggerated.

The implication is that opponents of state-sponsored regulation are guilty of wild hyperbole. But I do not forecast an apocalypse, merely a slow, stuttering descent into a version of controlled speech alien to the British tradition. But the Leveson Inquiry has made stark one conclusion that is profoundly inconvenient to proponents of regulation backed by statute. Virtually all the behaviour that has provoked demands for it is either actionable or contrary to criminal law.

Hacking of telephones and computers, harassment, data theft, forgery, breach of privacy or copyright are all covered by existing law. So is contempt of court.

There was, in a few national newspapers in the early years of this century, a culture devoid of ethics that regarded law-breaking as a tremendous way to pursue stories. Working with the encouragement of their editors, some reporters behaved as though they were above the law.

Appropriate remedies existed in law for all the offences these journalists committed. But the culture of the industry was such that it did nothing to draw their activities to the attention of the police.

This policy of culpable myopia reached a nadir when the Press Complaints Commission failed to effectively investigate hacking at the News of the World. But regulation could not have prevented the hacking scandal; this was a criminal matter.

So, no change is not an option. There must be effective regulation of the Press. A new self-regulatory system must have powers to investigate wrongdoing.

It must have the power to issue fines for unethical conduct and an absolute duty to inform the police if any evidence of criminality comes into its possession. Above all, it must be independent from government, parliament and state.

An officially regulated Press is an appalling idea. A few individuals who have our collective sympathy and who have received or will receive richly deserved compensation might enjoy the spectacle. But we would all be losers.